The present invention relates generally to electroluminescent devices and more particularly to use of an electroluminescent device as a document illumination source for an image input scanner.
An image input scanner is an apparatus, which is used in facsimile or copy machine or the like to scan the image of an original such as a document. The document scanner ordinarily employs a light source, an image sensor array, and an optical system for forming an image of an original on the image sensor. In use, a document on an opaque substrate is placed with the surface containing the original facing down on a flat transparent reference surface, typically glass. The original document is fixed on the surface, such that a single line of the original, hereinafter referred to as a xe2x80x9cscan linexe2x80x9d, is illuminated from below. The light reflected from the scan line is directed through an optical system to form an image of the scan line on a sensor such as a CCD array. The CCD array converts the optical signal to an electronic representation of the scan line, comprising a line of digital picture elements, or xe2x80x9cpixelsxe2x80x9d. The desired portion of the original is scanned, one scan line at a time, by moving the original relative to the illumination system, optical system, and sensor along a direction hereinafter referred to as the xe2x80x9cscanning axisxe2x80x9d. In some systems, the illumination system, optical system and sensor are configured to move together as a unit. In other systems, the original is moved while the illumination system, optical system and sensor remain fixed.
For a number of years, a preferred light source for scanning documents has been the fluorescent lamp. A fluorescent lamp has electrodes at each end of a tube containing a noble gas, such as argon. A large potential difference applied between the two electrodes at the ends of the lamp generates an electric field within the tube. Electrons emitted from the negative electrode collide with mercury and noble gas atoms as they accelerate toward the positive electrode. Collisions with noble gas atoms result in the electron traversing a zig-zag path toward the positive electrode, thus greatly increasing the probability of collision with a mercury atom. Electron collisions with mercury atoms result in excitation or ionization of the mercury atom; when the excited mercury atoms relax to their normal energy state, ultraviolet radiation is generated. The tube is coated by phosphors, which transform the incident ultraviolet radiation to visible light. Fluorescent lamps are desirable to use as a light source because they are energy efficient and operate at a low temperature. However, the fluorescent lamp is an inherently unstable light source. It is an arc lamp with light output highly dependent on the localized temperature dynamics of the arc, the noble gas and the vaporized mercury. Consequently, the light intensity from the lamp varies both spatially and temporally along the length of the lamp. Such variation degrades the quality of scanned images. To address this shortcoming, fluorescent lamps are commonly warmed prior to use, such that the heat generated from the arc has minimal impact on spatial uniformity and temporal stability. This adds mass and complexity to an already bulky illumination system, which may require the addition of shielding to protect the scanner sensor from heat and stray light.
These problems are intensified in a color scanner, which typically requires broadband illumination over three spectral ranges in order to accurately reproduce an image of the original. Fluorescent lamps are commonly broadband sources, however the phosphors are generally selected to irradiate in the red, green or blue region of the visible spectrum. While it is possible to mix all three into a single blend, the phosphors age at different rates, which results in varying color fidelity of the scanner as the lamp ages.
In one prior art embodiment of the color scanner, three different broadband illuminators are sequentially shown onto a color. Reflections from each illuminator are measured to reconstruct the color of the area. In another prior art embodiment, the three lamps are placed into an optical system such that they simultaneously illuminate a common scan line on an object, with the reflected light measured by a sensor. These systems tend to be substantially larger, wasteful and more complicated than a single lamp approach. In addition, stray light and heat problems increase three-fold.
In other color scanners, a single xe2x80x9cwhite lightxe2x80x9d fluorescent lamp is the illumination source. In these devices, the reflected beam is split into different paths to be measured by sensors that are sensitive to different colors. The difference in spectral sensitivity may be achieved by placing color filters over the same type of sensors. This method again incurs the weaknesses of a fluorescent lamp.
One solution to these weaknesses is presented in U.S. Pat. No. 5,525,866 to Mueller, et al., which discloses an edge-emitting electroluminescent device for illumination in a scanner. An electroluminescent device provides a broadband, directional, solid-state source that is stable, spatially and temporally uniform, rugged, efficient, compact, requires a minimal warm-up period and may be single or multi-colored. In Mueller et al., the device is fabricated such that light is emitted from an edge of the device in narrow lines, rather than from the device surface. However, a narrow line source may have limitations for scanner use. Typically, the medium scanned is planar; as light shines onto the planar medium, diffused light will be reflected and measured by a sensor. However, in many situations, the medium scanned may have a curved surface, such as bound printed subject matter close to the cusp of two adjacent pages. Due to the curvature of the medium, the light may impinge on a position in the curvature quite different from the desired planar position, thus reducing the amount of reflected diffused light reaching the sensor. One approach to resolving this problem is to increase the power of the source and to widen the beam-width of the radiation through optics. However, this will increase the complexity and cost of the scanner.
Another solution is presented in U.S. Pat. No. 5,598,067 to Vincent, et al., which discloses a surface-emitting electroluminescent device for illumination in a scanner. In the device of Vincent, et al., the device is fabricated such that it emits light that is both spatially and temporally homogeneous from a surface rather than an edge. Multiple such elements may be combined into groups, with the radiation of the elements from each group centering around a frequency that is different from those of the other groups. The elements may be electronically controlled so that elements from each group emit radiation together, and the emission from each group is done in a sequential manner. Alternatively, all the elements emit together, to produce a desired spectral power distribution. However, electroluminescent devices are a pulsed light source. If the integration time of the image sensor is short compared to the duration of the light pulse, portions of the original will be scanned under conditions of relatively low illumination, degrading the accuracy of the scanned images.
Accordingly, the present invention satisfies the need for a broadband illumination source that is spatially and temporally uniform over the integration time of the image sensor in a scanner.
An electroluminescent device used as an illuminating source in a scanner, including a plurality of electroluminescent elements, each having a transparent electrode with a top surface, a radiation generating stack under the transparent electrode, and a second electrode under the radiation generating stack. A voltage source having a plurality of phase characteristics is coupled across the electrodes of each of the electroluminescent elements to apply a voltage to each of the elements.